PROJECT SUMMARY Opioid abuse is a serious global problem that affects the health, social and economic welfare of all societies. Opioid use disorder (OUD) is a medical condition characterized by the compulsive use of opioids despite adverse consequences from continued use and the development of a withdrawal syndrome when opioid use ends. OUD involves both addiction to and dependence upon opioids. Opioids include prescription pharmaceuticals such as morphine, codeine, oxycodone and hydrocodone as well as illicit drugs such as heroin. Although animal models provide a rigorous, convenient means to precisely control environmental context and drug exposure, and assess behavioral, genomic, biochemical and cognitive changes, effective utilization of such models as an efficient proxy for human addiction remains challenging. The identities of gene variants that mediate behavioral differences in laboratory animals remain largely unknown, greatly limiting interpretation of physiologic differences and understanding of the environmental effects on drug abuse, and hindering translation of genetic findings to humans. This limitation highlights the urgent need for an integrated genomic characterization of a robust animal model of opioid abuse. The goal of this project is to exploit an outbred animal model of opioid abuse research, coupled with comprehensive genomic characterization, to improve detection of the underlying phenotypic and genomic changes associated with transition to opioid abuse. The project will engage an international multidisciplinary team of experts in the areas of addiction science and behavioral studies, neurobiology, rodent genetics, computational genomics, bioinformatics, and high throughput computing and data modeling to accomplish three Specific Aims. In Aim 1, we will exploit a validated outbred animal model of heroin self- administration with extended access that produces behavioral phenotypes consistent with increased drug consumption, accelerated motivation for drug intake and elevated drug-seeking in periods of drug absence. Animals at two distinct geographical sites (one in the USA and one in Europe) will be stratified across a ?vulnerable? ? ?resistant? spectrum with the objective of performing genetic screening on all individuals. In Aim 2, we will carry out a genome-wide association study (GWAS) using genotyping by sequencing (GBS) on 1,000 animals sampled from the two locations. Validation of the most significant genetic variants will be performed in an independent validation cohort of 100 animals. In Aim 3, we will assess the genomic impact of the genetic variants uncovered in Aim 2 via high throughput RNA sequencing to assess gene expression of relevant functional genes and uncover expression quantitative trait loci (eQTLs). Chromosome conformation capture (3C) will be used to physically link an eQTL with its target gene assigning causality to a variant and its regulated gene.